For approximately 200 years coal has been an important natural resource utilized for various purposes including heating, smelting, coking and the production of electrical energy.
Coal is formed primarily of vegetable matter or plant material that is decomposed under the influence of pressure, temperature and moisture, without access to air. This process results in a change in both the physical and chemical properties of the plant material. It is generally agreed that peat represents the initial, unconsolidated stage in the development of coal. Development continues with time and the application of temperature and pressure to lignite, sub-bituminous, bituminous and anthracite.
The chemical properties of coal are based upon its chemical constituents. The constituents are largely determined by several factors including the type of vegetation from which the coal was originally formed, the extent to which decay was permitted to proceed, the pressure to which the decaying vegetation was subjected, the foreign matter that was deposited onto the decaying vegetation while the vegetation was being converted into coal and the foreign matter that infiltrated in solution after the coal was formed. Additionally, the heat to which the decaying vegetation was subjected is of importance.
The major constituents of coal are carbon, hydrogen, nitrogen, sulfur and oxygen. The proportion of any of these varies from coal seam to coal seam and, to a lesser extent, in the same coal seam.
The amount of carbon in coal increases with rank from lignite at the low end to anthracite at the high end. It is the carbon content of the coal that supplies most of its heating value. The hydrogen content of coal generally ranges between 4.5 and 5.5 percent. This hydrogen also supplies some of the heating value. Oxygen exists in coal in several forms. The oxygen content is useful in coking and in gasificttion and liquefaction of coal. Upon combustion, the carbon, hydrogen and oxygen constituents of the coal combine to produce water vapor, carbon dioxide and some traces of carbon monoxide.
Nitrogen is present in coal almost exclusively in organic combination in percentages up to about three percent. In combustion, the coal nitrogen is converted primarily to elemental nitrogen, ammonia and small amounts of nitrogen oxides.
Of far greater environmental concern is the presence of sulfur in coal and the combustion products produced as a result of that sulfur. More particularly, it is believed that the sulfur byproducts from coal combustion, and sulfur dioxide (SO.sub.2) in particular, are responsible for acidifying rainfall to a pH in the range of 3.5 to 4.5. This acid rain damages vehicles, buildings and other personal property. It also collects in lakes and streams lowering the pH level of those bodies of water and, in some cases, adversely affecting those ecosystems.
As a result of these environmental concerns, the federal government has issued regulations within the New Source Performance Standards, NSPS, that limit sulfur dioxide emissions from power production facilities. As a substantial portion of the coal reserves in the United States, and particularly the primary coal reserves located in Kentucky, Illinois, Indiana, Ohio and West Virginia, have relatively high sulfur levels, combustion of these coals will exceed the proposed NSPS standards of sulfur dioxide emissions of less than 1.2 lbs/MMBtu. Accordingly, a need exists for effectively and inexpensively controlling sulfur dioxide emissions in order to allow the continued utilization of this relatively high sulfur eastern coal. This goal may be accomplished by one of two approaches. The first is pre-combustion cleaning of the coal. The second is post-combustion cleanup of the combustion flue gas.
In post-combustion clean up or flue gas scrubbing a water-lime or other water-alkaline slurry is often used to scrub or remove sulfur dioxide from the flue gas before it is emitted at the stack into the atmosphere. The product of this scrubbing is a sludge with an insignificant commercial value, that has to be de-watered and is land filled. Accordingly, this is an expensive process that merely changes the form of the sulfur dioxide environmental hazard but does not eliminate it. In contrast, the present invention relates to a novel and particularly effective process for the pre-combustion removal of sulfur from coal that produces commercially useful products while essentially eliminating the environmental hazard.